1. Field of the Invention
This invention relates to the art of magnetic recording and more particularly, to magnetic recording mediums such as magnetic tapes, magnetic sheets, magnetic cards, magnetic disks or magnetic drums for use in video, computer and audio systems.
2. Description of the Prior Art
Magnetic tapes for use in magnetic recording and reproducing apparatus such as, for example, video tape recorders should have good traveling performance as well as good electromagnetic conversion characteristics because they are run at high speed while maintaining physical contact with a number of parts such as tape guides, magnetic heads and the like. More particularly, the tapes should have not only good smoothness, but also a small coefficient of dynamic friction in order to ensure smooth and stable runnability.
In particular, recent video tape recorders have functions of slow reproduction, frame-by-frame reproduction, fast reproduction, and the like, so that more improved runnability of the tape is demanded along with the durability of the magnetic layer.
In order to improve the runnability, it is usual to add lubricants to the magnetic layer. A great number of fatty acids, fatty acid esters and mixtures thereof are frequently employed as the lubricants for these purposes. These types of lubricants are described in a number of patents such as, for example, U.S. Pat. Nos. 3,704,152 and 4,002,804 and Japanese Laid-open Patent Application Nos. 58-141440 and 58-158032. However, when fatty acids alone are used as the lubricant, the durability at the time of the frame freezing operation is not so good. On the other hand, fatty acid esters tend to exude, causing blooming of the esters during storage.
The above-indicated Japanese Laid-open Patent Application No. 58-158032 describes magnetic recording mediums which make use of lubricants consisting of mixtures of fatty acids and fatty acids esters. Mentioned as such fatty acids are lauric acid, palmitic acid, myristic acid, stearic acid and the like, and as fatty acid esters, esters of monobasic fatty acids and monohydric alcohols such as ethyl stearate, butyl stearate, amyl stearate, butyl palmitate, butyl laurate and the like. This medium is stated as having improved durability and strength.
However, the Japanese Laid-open Patent Application No. 58-141440 describes lubricants which comprise mixtures of fatty acids and fatty acid esters, to which extreme-pressure agents are added. It is stated in this patent application that the extreme-pressure agents serve to improve wear resistance of a magnetic layer. The extreme-pressure agents mentioned are, for example, phosphorus compounds, sulfur compounds, halogen compounds, and organic metal compounds.
As will be appreciated from the above, mixtures of fatty acids and fatty acid esters are not considered to be satisfactory with regard to durability and/or wear resistance.